Add PyPy to the Travis build

[hugovk]

See #570

All that was needed was a "sudo apt-get install python-tk" in the .travis.yml.

But perhaps the install should only happen for the PyPy build, what do you think?

And should PyPy be an allowed failure or would you prefer a normal build?

[kmike]

I think that it is a good change, but it still leaves Pillow's detection of Tkinter broken, and masks this issue by installing tkinter. For PyPy users without tkinter Pillow will fail. See travis-ci/travis-ci#1654.

[wiredfool]

I'm open to changing the try/except in setup.py to check OSError as well. That should fix tk detection on pypy, if I'm reading the travis thread correctly.

[wiredfool]

It looks like pypy isn't getting the tkinter package on travis. I suspect that it's a combination of site packages only working with py27 (the default python) and the python-tk package installing for that library.

Also, there's one test that's slower than a slug in winter on pypy -- see #484. If we push this to master, we should fix or disable that test.

[aclark4life]

Thanks

[aclark4life]

Fixes #570